WHAT'S YOUR SIGN? THE SCIENCE BEHIND THE ZODIAC

An Introduction...

Concept:

Motions of Celestial Objects:

At the earliest time of human existence, no one was aware that Earth was
rotating around its axis. Early astronomers watched the sky and saw
various cyclical motions of celestial objects such as the Sun, Moon, stars, and
planets that had periods or cycle times
ranging from 24 hours to a year. A natural
assumption that resulted from observations was that the reason why
the Sun, Moon, planets
and stars rose and set every day was due to their own motion around
the Earth. Now we know that these so-called diurnal or daily motions
are the result of the Earth's own rotation or spin about its North-South axis
with a period of 24 hours.

In early times, objects in the sky were believed to be affixed
to transparent celestial spheres
and their motions were thought to be a result of the motion of these
spheres as they revolved around the Earth.
Groups of bright stars were observed to form prominent
patterns in the night sky called Constellations, which have been
historically ascribed to mythological figures, such as Orion the Hunter,
and Gemini the Twins. The early
astronomers recognized that constellations "appeared" and "disappeared" with
the change of the seasons throughout the course of a year.
The Sun, Moon and planets were observed to move in relation
to the fixed background of stars, or constellations.
Because of their perspective from Earth,
early astronomers observed that all the planets visible with the
naked eye as well as the Sun seemed to
pass in the course of a year
through a region in the sky occupied by
twelve specific constellations.

More than 3000 years ago, astronomers wondered how the sky would appear if the
stars could be seen during the daytime. Based on their observations of the
night sky, some astronomers determined that during the daytime, the Sun would
appear to "enter" or pass through
a different constellation each month. These twelve
constellations are called the Zodiac. Many ancient people believed
that a person's behavior, emotions, and fate were heavily influenced
by the month of that person's birth--ie. that person's astrological
sign. Even today, thousands
of years later, some people still believe in the powers of the zodiac and
read their horoscopes for daily guidance. However, many people read their
horoscopes for fun, not because they believe in predictions.

Ecliptic, Celestial Equator, Equinoxes, and Precession:

While making its daily rotations around its axis, the Earth is also making
yearly revolutions as it orbits around the Sun.
The Earth travels in space as it revolves about
the Sun in a planar orbit that is approximately circular.
This motion of the Earth makes the Sun appear as if it is moving
eastward in the course of a year against the background of
the zodiac constellations. This apparent motion of the Sun
is caused by our changing point of view or viewing direction as we observe
from the surface of the Earth as it moves in its orbit. The path in the
sky defined by this apparent motion of the Sun (which is actually the plane
of the Earth's revolution about the Sun) is called The Plane of the
Ecliptic .

It turns out that most of the other planets in the solar system also revolve
around the Sun in orbits that lie approximately along the same Ecliptic
plane. Try to imagine
that the Sun sits in a hole in the middle of a large, round cake pan. If
the planets were marbles, they would roll around the Sun only on the
surface of the cake pan and would not move to another pathway or
to a higher or lower level.
It turns out that one of the nine planets does not orbit
the Sun in plan that lies close to the Ecliptic, and this is
Pluto, the farthest planet from the Sun.
Pluto's orbit lies about 17.5 degrees out of the Plane
of the Ecliptic. This orbit causes Pluto to lie either above or below
the Plane of the Ecliptic most of the time.
Figure 1 below shows
a diagram of the Earth orbiting the Sun along the Ecliptic,
as well as the other solar system planets orbiting the Sun approximately
along the plane of the Ecliptic.
Pluto's orbit is not
included in Figure 1, so you may well want to challenge your students
to draw Pluto's orbit on the diagram.

The Zodiac constellations, as envisioned by ancient
astronomers, were ascribed specific patterns
that resemble the shapes animals, half-animals, and human
beings. The Constellations of the Zodiac actually form
an imaginary belt in the sky that extends about eight degrees
above and below the Plane of the Ecliptic. In Figure 2 below, The
Constellations of the Zodiac, we are looking at the Earth
from far out in space. We must keep in mind as we look at the
names of the zodiac constellations that at any given time of the
year, the Sun is between Earth and one of these constellations.

We are familiar with the Earth's Equator because of our knowledge of
geography. If we could extend the earth's equator into space so that
it could be viewed against the background of stars, we'd be able
to see what modern astronomers call the Celestial Equator.
Because the Earth's axis of rotation is tilted with respect to
the Ecliptic (or orbital plane of the Earth about the Sun) by 23.5 degrees,
the Celestial Equator and the Ecliptic do not lie on the same plane, but
cross each other at an angle of 23.5 degrees. The two points in the sky
where these two planes (Celestial Equator and Ecliptic) cross
are called the Equinoxes.
We call the Vernal Equinox
the intersection point where the Sun, in its apparent motion against the
background stars along the Ecliptic, crosses the Celestial Equator from
south to north, usually occurring around March 21.
Similarly, we call the Autumnal Equinox
the intersection point where the Sun, in its apparent motion against the
background stars along the Ecliptic, crosses the Celestial Equator from
north to south, usually occurring around September 21.
The first day of Spring then corresponds to the Vernal Equinox
and the first day fo Fall corresponds to the Autumnal Equinox.
During the time of the Equinoxes, we on the Earth experience
twelve hours of day and twelve hours of night.

The Earth's rotation on its axis has caused the Earth's shape to
diverge from a sphere, and has caused the Earth's equatorial regions
to bulge out, in the same way that a skater's skirt spreads outward
as she spins rapidly on the ice. Because the Earth's equator is
tilted with respect to the orbital plane of the Earth around the Sun,
the so-called Ecliptic plane, the Earth's equatorial bulge is also tilted
with respect of the plane along which the Sun and Moon travel. The
Moon and the Sun exert a gravitational "tug" on the Earth's equatorial bulge,
trying to pull the Earth's equatorial region to be aligned with the
Ecliptic plane. This tug, along with the rotational motion of the Earth on
its axis, the revolution of the Earth around the Sun, and the
revolution of the Moon about the Earth, cause the Earth to wobble about
its axis of rotation, similar to the motion of a spinning top.
This motion is called Precession.
An extension of the Earth's axis out into space traces out a conical figure
with a time cycle or period of 26,000 years.
Figure 3 below shows a schematic of the Earth's precession.
The Earth's precession implies that although
Polaris is currently the star above our North pole, in about 13,000
years Vega will become our North star; only, after yet another
13,000 years, will our North pole will once again point towards
Polaris.

Equinoxes, Precession, and Astrology:

According to astrology, a person's sign is determined by the position
of the Sun on the date they were born. Namely, a person is born under the
sign of Virgo if the constellation Virgo was behind the Sun at the time
of birth. Thus, a person can see their sign constellation approximately
6 months after they are born, since we must wait for the Earth to
travel to the other side of the Sun to see the constellation at night.

According to astrological traditions, the Vernal Equinox is the day
that the Sun "enters" the constellation Aries. However, this is not
always the case, as astronomical observations have found. It
turns out that the Earth's precession makes this
astrological prediction wrong.
The Earth's precession causes the Equinoxes to move with respect to
the background constellations of the Zodiac.
Because of the Earth's precession, the
dates of the Equinoxes gradually change over a 26,000 year cycle. This
means that currently the constellation which is behind the Sun, is
actually different from the one predicted by astrologers. For
example, according to traditional astrology, the Sun is "in Gemini,
the Twins" between May 22 and June 21; but when a person looks
at the nighttime sky, the constellation of Gemini is easily visible
above the western horizon after sunset all through the month of
May until around June 20.

Theme:

Tied to the California Science Framework:

PATTERNS of CHANGE and STABILITY

SYSTEMS and INTERACTIONS

Objectives:

The students will be able to describe what is meant by the Plane of the
Ecliptic.

The students will be able to recognize and identify the constellations of the Zodiac.

The students will be able to explain why the zodiacal constellation
"occupied" by the Sun is not visible in the sky.

The students will be able to explain why the location of the Sun in the
traditional astrological signs does not match the observable position of the Sun in the sky.

Grade Level and Prerequisites:

Grade Level: 7 - 12

Pre-requisites:

Students must have an understanding of Earth's rotation as the cause of day and night.